Heat exchangers transfer heat energy from one fluid (liquid or gas) to another without permitting the two fluids to come into direct contact with one another. There are three main types of heat exchangers, defined by their construction or body types: shell and tube, plate, and air-cooled. Shell and tube (or tubular) heat exchangers are used in applications where high temperature and pressure demands are significant and are the type most commonly encountered in petroleum refineries and petrochemical plants, largely because of their ability to meet the severe service requirements. Tubular heat exchangers are also employed when fluid contains particles that would block the channels of a plate heat exchanger.
Plate heat exchangers are often used for service with low viscosity fluids, including liquid-to-gas heat exchange. Usually, the service requirements impose only moderate demands in terms of operating temperatures and pressures. There are several types of plate heat exchangers including gasketed, brazed, welded and semi-weld or hybrid types. Semi-welded or hybrid type plate exchangers also exist, with plates that are welded together in pairs to allow one fluid to flow in a channel formed by a pair of plates which are welded together with the other fluid passing in a gasketed channel between the welded pair.
Air-cooled heat exchangers have an integral powered fan for cooling the fluid passing through the exchanger, as in an automobile radiator.
As noted above, the shell and tube type exchanger is commonly encountered in petroleum refinery and chemical plant service because of its ability to meet severe service specifications, especially in terms of temperature and pressure. There are several types of shell and tube heat exchangers including U-tube, straight and spiral designs. The U-tube design consists of tubes bent into a U-shape bundle which is fitted with a header to direct the fluid into the tube bundle; supports or flow baffles may be used to direct the fluid flow around the outside the tubes. The straight-tube design with opposed header assemblies is favored when heavy fouling is likely to be encountered in operation: the head assemblies can be removed and the straight line tubes can be mechanically cleaned.
Petroleum refineries and petrochemical plants suffer high operating costs from lost heat transfer efficiency, energy reduction, and cleaning as a result of fouling that occurs during the thermal processing of whole crude oils, and other media in heat transfer equipment. Estimates of the costs associated with exchanger fouling are in the billions of dollars per year for the petroleum refining industry. While there are many types of refinery equipment affected by fouling, more recent cost estimates have shown that the majority of profit losses occur due to the fouling of whole crude oils and blends in the exchangers of the preheat trains preceding crude units.
While heat exchanger fouling can occur by different mechanisms, one of the more common root causes is asphaltene precipitation and adherence of the precipitates at hot surfaces. To return the unit to more profitable levels, such fouled heat exchangers typically need to be removed from service and cleaned.
The current approach to clean fouled heat exchangers involves isolating the exchanger for cleaning off-line. This is typically performed once the exchanger's efficiency is reduced to non-profitable levels as a result of the coke build up. Coke is a fouling product that is difficult to remove from surfaces by chemical means due to its insolubility in many solvents. To carry out the cleaning, the exchanger is first drained and the coke deposits removed by mechanical typically using brushes or darts to remove the deposits although alternatives such as by liquid lancing with high pressure jets or with abrasive liquids or by blasting with solid carbon dioxide have been explored but all these expedients have the same disadvantage, that the exchanger has to be taken off-line to be drained so as to obtain access to the open ends of the tubes in the tube header. So, regardless of the actual technique used to remove the fouling from the tubes, there is a major loss from the down time resulting from the loss of use of the equipment as well as from the necessity to have to re-route the process liquid to other exchangers so as to allow permit continued heating of the process fluid and operation of the process unit. It would, therefore be desirable to devise a method which will reduce or eliminate the need to physically remove and clean affected heat exchangers.